Have high blood pressure. Weakness, bubbles under heels when walking. Could it be anything else?

Hello, my mother is 87 yrs old, she is very fit, the only medical condition she has is high blood pressure , which she takes daily tablets for, she has recently seen her GP and all is ok, but in the last week she has been feeling slightly weak, although it has been very hot here, and she had been sewing, and she said that she felt like she had bubbles under he heels when walking, anyway i sat her down in an air con place and she rested and felt fine, could it be anything else? or was it just heat and tiredness